1. Field of the Invention
This invention relates to supported silver epoxidation catalysts and more particularly to a method of preparing a stable, activated silver epoxidation catalyst from specific silver carboxylate/amine complexes.
2. Description of the Prior Art
Supported silver catalysts have long been used for the air oxidation of ethylene and more recently in a so-called "oxygen process." Although the first reference to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise De Catalyse Generalisee, French Pat. No. 729,952 (1932); and Lefort, U.S. Pat. No. 1,998,878 (1935).
Since silver is expensive, optimizing the amount of silver employed in a supported catalyst for a desired conversion and selectivity to products has been widely investigated. A variety of techniques have been developed for the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver supports such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949) at 813-827, describe a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60.degree. C. and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962, to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface.
Recently it has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver oxalate impregnating solution which employs a solubilizing/reducing agent of ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylenediamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decomposition gives supported silver catalysts for ethylene epoxidation.
It has now been discovered that extremely stable, physically durable, supported silver catalysts can be simply produced by impregnating a porous, inorganic substrate with a complex formed by dissolving a silver carboxylate in certain amines and thermally decomposing the complex to deposit the silver on the substrate and activate the silver. Surprisingly, the supported catalysts of the instant invention show high productivity and C. more active at a given reactor temperature than the prior art catalysts. Additionally, the instant inventive catalysts show high attrition resistance and surprisingly high mechanical strength. It has further been found that the instant catalysts provide good selectivity in air oxidation processes. This is particularly important in that such processes are not closed systems and some proportion of the unreacted ethylene is lost by venting excess gas.